Hello (and thanks!) from Wisconsin

[stru0121]

Spencer here, located just outside Madison, WI. I recently bought a 1981 GS850G (25k miles). Sharing notes in case others see similar characteristics with new bikes. I'm a novice with motorcycle repair so bear with me if I misuse some terms/steps.

Bought the bike "trusting a stranger" and based on general condition of the bike. Had a slow oil drip. Previous owner said it "ran great" before he put it away but might need a shim adjustment.

Started by replacing the battery, changing the oil (added an aluminum crush gasket), then started to see what general condition it was in. At this point the bike idled well. When I took it out on the road (about half a mile), it was noticeably smoky and lacked power - hit or miss when trying to accelerate. Got home fine but not ready to ride.

Let it sit over the winter, then got a carb rebuild kit to clean up the carbs (thinking that was part of the choppy acceleration). Found that the carb for cylinder #2 had a stripped out air/fuel adjustment screw. Also, philips-head screws were seized up for one of the mounting bars (no luck with PB Blaster but yet to try the torch). Ordered a new set of carbs off eBay for ~$200, thinking I'd need the adjustment screw. Yet to install them, though.

Next, tested compression. #1: 95psi. #2: 80psi. #3: 75psi. #4: 95psi. Same result with wet test (re-test after adding ~tablespoon of oil to the cylinder). Chatted with friends and read about this scenario, sounds like it will probably need a new head gasket. Also read that this could be leaky valve guide oil seals, which I can replace at the same time.

Meanwhile, the slow drip of oil still comes from the oil drain plug. In hindsight, it might be helped by a copper crush washer instead of aluminum (Cu being more malleable). TBD.

Also added household weather stripping to one of the air box covers because the gasket was broken in two spots.

Also disassembled the fuel valve/petcock because it developed a slow drop after I took off the fuel tank. The large gasket between the petcock assembly and fuel tank looked a little flat, so I ordered a new one - decided to get it off eBay because only the full rebuild kit was available on motorcycle part sites.

At this point, my next step is to take off the cylinder head, inspect, and replace gaskets. Found a couple of good YouTube videos walking through the clean-up process (famous last words?). We'll see about the compression after that! Not sure how this forum is typically used but I'll post an update after I get everything back to good.
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[LAB3]

Sounds like you're on your way! Just a heads up on the petcock, there are fiber washers on the heads of the mounting screws, be sure to replace those. You can get them for pennies at a local hardware store, there's an English size that's just a hair too small. Roll up a small piece of sandpaper and ream out the hole just a bit to make them fit, that's probably the source of your leak

 

[Rich82GS750TZ]

Welcome. You're in the right place.
If you did the compression test right, with the throttle held wide open, then your numbers are below service limit (if memory serves, that's around100 psi for most models). If you're going to take the head off and replace valve seals, lap the valves, and just do new rings too. You're already in there. Since you're taking the head off, you also need to remove the cylinders and replace base gasket. Use OEM gaskets if ast all possible.

Lots of advice for a first timers in this thread. It's where you should look before doing anything else?
https://www.thegsresources.com/_for...tenance/46923-top-10-newbie-mistakes?t=171846

Factory Service Manuals, wiring diagrams, and so much more can be found on BikeCliff's Website:
https://gsarchive.bwringer.com/

 

[LAB3]

Agreed that the compression is awfully low. I've often heard this is common for a bike that's been sitting a long time and you really need to get the temps up before getting a "true" reading. If it where me I'd get it running well enough for a good little ride THEN do the compression check. I'm not sure that a head gasket or head rebuild is going to do much based on the info you've already given other than make sure you have the valve shims set properly before making the decision to pull the head

 

[Rich82GS750TZ]

Absolutely, do the simple things first. Get your valve clearance in spec first. Of course, if that doesn't alleviate your poor, smoky performance, you'll need to measure and re-shim when you get the rebuilt head back on.

 

[gtem]

When I was working through some carb issues on a GS650G, I was seeking some scary looking low compression readings on cyl 1, but it was all down to cylinder washdown due to a carb issue. Statistically speaking with GS motors, I'd say it's fairly unlikely (not impossible) that you have a "real" compression issue at this point. Smoky exhaust is also something you'd see if you were dumping a ton of fuel, whether via leaky float valves or general misadjustment. I'd focus your efforts on getting the carbs right first, either your set or the one you bought or some combination of the two. A word of caution, make sure to check out https://gsarchive.bwringer.com/bikecliff/images/carbspec_float_height.html there were oftentimes year-to-year differences on the BS34 Mikunis so keep that in mind. Don't use any brass (jets) out of a cheap rebuild kit. Chances are, your old ones are fine, if you do need new jets make sure they are genuine Mikuni. Check valve clearances, then see where you land. Boroscopes are cheap nowadays, stick one down the sparkplug tubes and see what the cylinder walls look like. Only then would I start tearing into the motor.

 

[stru0121]

Appreciate all of your thoughts! A few updates and additional notes:
- I performed the compression test with carbs removed, hence my suspicion of valves / gaskets / rings.
- I tested valve clearance and some are within spec (minimum feeler is a snug fit), but others are too tight for the minimum in-spec size of 0.03. Seems like I need new shims for those valves and maybe this is at least part of the low compression issue on cylinders 2 and 3.
- I don't think I mentioned this previously, but there is no visible oil leakage out of the head gasket or base gasket.
- I disassembled the petcock out of curiosity. Gaskets all seemed to be in good condition, so I may need to look more carefully for the source of the drip and try the suggested washers.
- Rookie note: I mentioned seized screws for both the carb mounting bracket and carb boots. I got an impact driver (manual kind) and it works like magic to remove the stuck screws. Now I know.
- Rookie note: the slow oil drop from the drain plug was bugging me. Persisted after a copper crush washer, too. Added some teflon tape to the drain plug and no more drips as of ~3 days later, so seems to be resolved.

Again, thanks everybody for your thoughts. This forum is really valuable - appreciate everyone's insights and useful history.​

 

[stru0121]

Latest updates/progress:

• Got my valve clearances into spec, and compression is much better now - 105+ for cylinders 1, 2, and 4 (previously compression was around 90, 75, and 90 for those three cylinders).
• Surprisingly, cylinder 3 is still around 75-80 psi.
  • Intake valve clearance was 0.06mm after I replaced the shim and I had turned the crank manually a few times before putting covers back on.

I plan to take the cover off again and re-check cylinder 3 in case shims weren't seated properly. Fingers crossed!

Rookie note: when I planned my shim replacements, I made a spreadsheet to determine the new shim thickness needed to get to 0.08mm (max clearance range). In hindsight, I should not have assumed "0.02mm" as my input value for the "current shim thickness" in cases where I knew valve clearance was less than the minimum spec range of 0.03mm (also my minimum thickness feeler). Doing this meant I had a couple cases where I thought I would be around 0.07mm but after replacing, was closer to 0.04 or 0.03mm.

Rookie note: I was able to take shims from some valves to use for other valves, and went to eBay to buy the rest that I needed. Happy to see there's a lot of 29.5mm diameter shims available on eBay (although not cheap). I saw prior mention of the exchange process on this forum - seemed like that went away, but smart idea!